Regarding a mold for casting a metal, an alloy, or the like, there may be cases where the strength of the mold is too high when a casting is taken out and it is difficult to separate the mold. When the casting is taken out of the mold, the mold may be destroyed by an impact of a hammer, sandblasting, steel shot, or the like. Therefore, there is a possibility that damage from the impact caused by destroying the mold may be imparted to the casting and defects may be generated.
As a mold, a mold which is formed by mixing zircon, alumina, or the like with a silica sol and baking the mixture is known. Generally, such a mold is less likely to contract due to temperature decrease, and the coefficient of linear expansion thereof has a value different from that of a metal for the casting by one order of magnitude. Therefore, when the casting is cooled, a tensile stress is exerted on the casting due to contraction, and there is a possibility that defects such as cracks may be generated in the casting.
In PTL 1, a technique in which a mold is formed by using a material containing 10 wt % or more of zirconia is suggested. In the technique of PTL 1, the property of zirconia, in which the crystal structure thereof transitions according to temperature is used. That is, by using a mold, which is increased in temperature as molten metal is poured, a countless number of fine cracks are generated in the mold, which causes the mold to collapse in on itself.